IPT systems are a well known area of established technology (for example, wireless charging of electric toothbrushes) and developing technology (for example, wireless charging of handheld devices on a ‘charging mat’). Typically, a primary side or transmitter generates a time-varying magnetic field with a transmitting coil or coils. This magnetic field induces an alternating current in a suitable receiving coil that can then be used to charge a battery, or power a device or other load. In some instances, the transmitter coils or the receiver coils may be connected with capacitors to create a resonant circuit, which can increase power throughput and efficiency at the corresponding resonant frequency.
A common problem with IPT systems is controlling when the transmitter should be powered and when the transmitter should be switched off. A further problem arises when a non-receiver is brought into the range of the transmitter, and an unwanted current (and therefore heat) is induced therein. These non-receivers are typically known as parasitic loads. Lastly, it may be possible to detect the presence of a receiver, but it may also be necessary to identify the receiver as being compatible with the particular transmitter. Attempting to transfer power to non-compatible receivers may result in inefficient power transfer (thus, undesired energy loss), or transmitter and/or receiver failure.
An obvious solution to the problems outlined above is to include a manually operated power switch with the transmitter. Though this provides a means for controlling when the transmitter should be powered, it undermines the convenience that is a goal of many IPT systems. It also requires a user to manually switch off the transmitter when the receiver is removed and does not accommodate any parasitic loads that may be introduced into the vicinity of the transmitter without the user's knowledge.
Automatic systems for the detection and identification of receivers have been described in the prior art. For example:                Systems that rely on contact-based interaction between the transmitter and receiver;        Systems that rely on communication signals sent between the transmitter and receiver; and        Systems that use non-radioproximity sensors (eg light sensors) to detect the physical presence of receivers.        
All of these approaches rely on additional componentry to implement the detection method. This adds complexity and cost to the design of IPT systems. Perhaps more importantly, they tend to add bulk, which frustrates attempts to incorporate IPT systems into smaller devices such as mobile phones, personal computers and the like.
To lessen these effects, it is known for IPT systems to utilise the power transfer componentry for detection and identification as well (ie multi-purpose).
The drawbacks of these approaches are:                The power transfer may need to be reduced or completely interrupted in order to carry out a detection method;        Where steady-state current is used an indicator of a receiver, unloaded receivers may erroneously give a false result;        May be sensitive to component variations and noise; and        May be unable to identify whether a detected receiver is compatible.        
It is an object of the invention to provide methods for detecting or identifying a receiver that do not require extensive additional componentry to that required for inductive power, that produce accurate results not sensitive to noise, that limit the time during which power is not being transferred, that can positively identify a receiver or to at least provide the public with a useful choice.